Quantum Chemical Study of 2-Mercaptoimidazole, 2
Transcript of Quantum Chemical Study of 2-Mercaptoimidazole, 2
Int. J. Electrochem. Sci., 6 (2011) 3729 - 3742
International Journal of
ELECTROCHEMICAL SCIENCE
www.electrochemsci.org
Quantum Chemical Study of 2-Mercaptoimidazole, 2-
Mercaptobenzimidazole, 2-Mercapto-5-Methylbenzimidazole
and 2-Mercapto-5-Nitrobenzimidazole as Corrosion Inhibitors
for Steel
M. Abreu-Quijano1, M. Palomar-Pardavé
1,*, A. Cuán
1, M. Romero-Romo
1, G. Negrón-Silva
2,
R. Álvarez-Bustamante1, A. Ramírez-López
1, H. Herrera-Hernández
1
1 Universidad Autónoma Metropolitana-Azcapotzalco, Departamento de Materiales, Av. San Pablo
180. CP. 02200, México D.F. México. 2 Universidad Autónoma Metropolitana-Azcapotzalco, Departamento de Ciencias Básicas, Av. San
Pablo 180. CP. 02200, México D.F. México. *E-mail: [email protected]
Received: 15 May 2011 / Accepted: 25 July 2011 / Published: 1 September 2011
In order to analyze the influence of substituent groups, both electron-donating and electron-attracting
and the number of -electrons on the corrosion inhibiting properties of organic molecules, a
theoretical quantum chemical study under vacuo and in the presence of water, using the Polarizable
Continuum Model (PCM), was carried out for four different molecules, bearing similar chemical
framework structure: 2-mercaptoimidazole (2MI), 2-mercaptobenzimidazole (2MBI), 2-mercapto-5-
methylbenzimidazole (2M5MBI), and 2-mercapto-5-nitrobenzimidazole (2M5NBI). From an
electrochemical study conducted previously in our group, (R. Álvarez-Bustamante, G. Negrón-Silva,
M. Abreu-Quijano, H. Herrera-Hernández, M. Romero-Romo, A. Cuán, M. Palomar-Pardavé.
Electrochim. Acta, 54, (2009) 539), it was found that the corrosion inhibition efficiency, IE, order
followed by the molecules tested was 2MI > 2MBI > 2M5MBI > 2M5NBI. Thus 2MI turned out to be
the best inhibitor. This fact strongly suggests that, contrary to a hitherto generally suggested notion, an
efficient corrosion inhibiting molecule neither requires to be a large one, nor possesses an extensive -
electrons number. In this work, from a theoretical study a correlation was found between EHOMO,
hardness (η), electron charge transfer (ΔN), electrophilicity (W), back-donation (ΔEBack-donation) and the
inhibition efficiency, IE. The negative values of EHOMO and the estimated value of the Standard Free
Gibbs energy for all the molecules (based on the calculated equilibrium constant) were negative,
indicating that the complete chemical processes in which the inhibitors are involved, occur
spontaneously.
Keywords: Steel; Corrosion inhibition; Quantum Chemistry; Sulphuric acid; 2-mercaptoimidazole;
adsorption
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1. INTRODUCTION
Recently, our research group has shown that both 2-mercaptobenzimidazole (2MBI) [1] and 2-
mercaptoimidazole (2MI) [2] can be very useful corrosion inhibitors for steel under highly acidic
conditions and pointed out that the effectiveness of the heterocyclic compound can be intimately
associated to its molecular structure.
Many efforts have been done to try to understand which are the basic features that grant the
corrosion inhibitory character to the organic compounds.
Theoretical studies at molecular level have been reported that aim at gaining insight on the
molecules’ chemical activity [3-7] and in particular to study corrosion inhibiting processes [8, 9] taking
into account model molecules and their structural and electronic properties.
According to some quantum-chemical results, a relationship has been found between corrosion
inhibition as the HOMO energy level increases [8-29], or as the HOMO-LUMO energy gap value
decreases within a group of organic inhibitors [10, 12, 28, 30, 31].
Some authors [11, 13, 19, 22, 24, 27] found that the polarity of the organic compound affects
the inhibitory character. Studies on chemical reactivity [6, 12-14, 17, 18] showed that the heteroatom
from the molecules plays an important role within the inhibiting mechanism leading to a better
understanding of this issue.
In this work, we studied four different small molecules which have similar chemical framework
structure but with different substituent groups or heteroatoms: 2-mercaptoimidazole (2MI), 2-
mercaptobenzoimidazole (2MBI) 2-mercapto-5-methylbenzimidazole (2M5MBI) and 2-mercapto-5-
nitrobenzimidazole (2M5NBI), using electrochemical techniques [2] and theoretical quantum chemical
calculations by applying the Density Functional Theory (DFT), in order to elucidate the inhibiting
activity of these molecules through the knowledge of their structural and electronic properties.
2. PROCEDURE
2.1. Quantum-chemical calculations methodology
The calculations have been performed with Gaussian03 [32] using the BeckeLYP functional
[33, 34] and the 6-311+G(d,p) basis set for both cases: in vacuo and with the Polarizable Continuum
Method (PCM) accounting for the solvent (water) effect [35, 36]. The PCM by Miertus et al. [35],
SCRF methods (self-consistent reaction field) were used to perform calculations in solution. These
methods model the solvent as a continuous with uniform dielectric constant (ε) and the solute is placed
in a cavity within it.
The calculations accounted for the complete set of electrons and the geometry of all involved
structures was fully optimized.
Computing was performed with a Pentium 4, 1.9 MHz and 500 Mb RAM workstation and
molecules’ visualization was carried out with Arguslab 4.0 commercial software [37-45] and the
Gausview 2.02 software [32].
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3. RESULTS AND DISCUSSION
3.1. Quantum-chemical background
The theoretical aspects of the inhibitor molecules performance have been studied to understand
and describe the experimental observations [2] based on the structural and electronic properties of
these inhibiting compounds.
To estimate the active sites pertaining to the different corrosion inhibitors considered in this
work, see Figure 1, and their inhibiting features, a set of thermodynamic parameters [42-45] was
devised from the Density Functional Theory [41], DFT, namely hardness (η), Fukui functions (f(r)) and
chemical potential ().
(a)
(c) (d)
(b)
Figure 1. Optimized molecular structures: (a) 2-mercaptoimidazole (2MI), (b) 2-
mercaptobenzoimidazole (2MBI), (c) 2-mercapto-5-methylbenzimidazole (2M5MBI) and (d)
2-mercapto-5-nitrobenzimidazole (2M5NBI).
The variables associated to DFT studies are the electronic density r divided in spin
contributions and , the systems’ energy )(),( rrE
and electron number (N), the
multiplicity, the external potential ((r)) and the magnetic field associated to the media (B(r))
dependent on Bohr’s magneton b, which is useful to define spin conditions. From these quantities the
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following values can be found: the energy (i(r)) of molecular orbitals’ (i(r)). In order to evaluate the
energy of the system, hence the electron density, the following functional expression was used:
)())()(()())()((
)(),()()())(()(),(2
21*
rrrrrrr
rr
BvBvd
rrErrJdrrE
bb
xcii
i (1)
where J is the coulombic energy functional and Exc is the exchange and correlation contribution
which permits evaluation of the electron interaction in the system, considering the interactions
amongst three or more bodies.
The differentiation process applied to the energy with a set of variables of the system, will
produce different thermodynamic response coefficients useful to predict a progress in a chemical
reaction. [42, 45, 46]
Particular attention is given to the response coefficients obtained after first differentiation,
which are associated to the chemical potential,N
, and the spin potential, s
:
N sN
rEN
)(
Nss N
rE
)(
(2)
where χ is the electronegativity of the molecule. The chemical potential can also be evaluated
in terms of the frontier orbital energies:
LUMOHOMO
NEE
2
1 (3)
From the second differentiation is possible to obtain the response coefficients associated to the
hardness:
EE LUMOHOMO
vNvN
NNN
ssN
rE
N
21
,,2
2
,,
)(
BB
(4)
Moreover, the directional components for a molecule (M) that accepts charge:
NN
N
cationMM
N N
N
NNN
s
)(
NN
M
N
anionM
N
N
NNN
s
)(
(5)
In a similar way electrophilicity index (W ) can be obtained from the definitions of global
hardness and chemical potential as follows:
NN
NW
2
2
(6)
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Furthermore, the following derivation process helps to obtain another important
thermodynamic response coefficient, the Fukui function:
vN
N
Nvf
r
rr (7)
Also, the generalized Fukui functions [42-46]:
vNNN
NNN
ss
Nvf
,,
r
rr
vN
s
NN
NsN
ssB N
f,,
1
r
rBr
vNsNN
sNs
Nvf
s ,,
r
rr
vNs
s
NN
sss
NBf
s ,,
1
r
rBr (8)
The analysis of the Fukui functions associated to a molecule is useful to obtain a term
associated with the ability of a system to modify its charge and/or multiplicity. To describe these
phenomena, the Fukui functions that best represent the system are:
s
cationMM
NsN
rrrf
)()(
)()(
s
ManionM
NsN
rrrf
)()(
)()( (9)
In this work’s case, both spin and charge changes were considered, through the following
approximations, because at the electrodes’ surface the changes of the electrical potential can reflect
charge changes associated to formation of anionic or cationic species. This also can be approximated
through the following expressions:
22
2
1)()()( rrrf
LUMOHOMONs
22
2
1)()()( rrrf
HOMOLUMONs (10)
A global parameter of the molecule such as the hardness (eq. 4) and a local description of the
molecule like the Fukui function (eq. 9 or eq. 10), were used to analyze the electron transfer processes
[42-50] to evaluate the sites where the molecule will receive charge (f +
(r)) when engaged by a
nucleophilic reagent or the preferred sites to donate charge (f -(r)) when engaged by an electrophilic
reagent. There are more detailed studies that can be used to determine the effect of a specific functional
group in a molecule [43, 47] or to make a most specific atomic analysis. From these studies the
condensed Fukui´s functions, also called Fukui´s indexes, were derived. To calculate such functions is
necessary to adopt a partition scheme of the electronic density among all the atoms of the molecule, to
associate a charge to each of them. In this situation, the electron density around an atom is associated
with its net charge. If Z i is the nuclear charge of the i-th atom, and )(r
i
N is the electron density
around it, then the net charge of the i-th atom of the molecule (M) is given by )(rZq i
Ndr
i
i
N and
finally the Fukui´s indexes are defined as [48-51]:
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)(cation
i
M
i
Mss qqf qq
i
Manion
i
Mssf
)( (11)
For the sake of notation simplicity, the set of thermodynamic parameters employed in this work
were defined as, for the hardness (eq. 4), )(rf for the Fukui function (eq. 10), and fi , for the Fukui
indexes (eq.11).
To estimate the hardness, the Fukui function and the Fukui´s indexes, it was important to
consider that the simulation process to evaluate the electron density must describe adequately the
system [7, 52]. Due to this fact, a Hartree-Fock-type density is meaningful, but including a correlation-
type density [29, 53].
3.2. Theoretical assessment
The results of the geometries’ optimization of the selected compounds are presented in Figure
1. The frameworks of these geometries show planar configurations for all the inhibitors.
Quantum-chemical indexes like EHOMO, ELUMO, ΔΕ (│EHOMO - ELUMO│) or hardness η, and
dipole moment are summarized in Table 1. To analyze the theoretical results obtained, it is necessary
to bear in mind that the trend for the experimental inhibitor’s efficiency was 2MI > 2MBI > 2M5MBI
> 2M5NBI. It is known that EHOMO is often related to electron donation ability of the inhibitor
molecules toward the metallic surface atoms, and it is expected that a higher EHOMO value would favor
a greater charge transfer [7, 10-14, 29, 54, 55].
Table 1. Values for EHOMO, ELUMO │EHOMO - ELUMO │(GAP), global hardness (η) dipole moment for
the inhibitors (in vacuo/with solvent) and total energy stabilization by solvent effect (ΔEsolv).
Inhibitor EHOMO
/eV
ELUMO
/eV
GAP
/eV
/eV
Dipole
/eV
ΔEsolv
/eV
2MI -5.09/-5.02 -1.16/-0.69 3.93/4.33 1.97/2.17 2.66/3.90 -0.72
2MBI -5.25/-5.21 -1.40/-1.38 3.86/3.82 1.93/1.91 2.40/3.86 -0.80
2M5MBI -5.13/-5.11 -1.32/-1.33 3.81/3.78 1.91/1.89 2.22/3.65 -0.78
2M5NBI -5.92/-5.61 -3.28/-3.55 2.64/2.07 1.32/1.04 7.44/11.61 -1.15
According to our results, the values of EHOMO show the following behavior 2MI > 2M5MBI >
2MBI > 2M5NBI for this property. In this case, the largest EHOMO corresponds to 2MI (see Table 1) in
line with the aforementioned experiments and the lowest EHOMO corresponds to 2M5NBI that in terms
of activity is the worst inhibitor. Some authors [50-55] have found that a smaller value of the hardness
is related to greater stability of the surface-inhibitor complex formed. Moreover, smaller values of
ELUMO show better capacity of the inhibitor to accept electrons. The trend obtained for the ELUMO
values was 2M5NBI < 2MBI < 2M5MBI < 2MI and for the hardness was 2MI > 2MBI > 2M5MBI >
2M5NBI, as it can be observed in Table 1. Some authors have reported a relationship between these
properties while others claim there is not such a relationship [54, 55]. Our results indicate no
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relationship between these properties, the hardness and ELUMO agrees with later works. It seems that
the larger the hardness value the greater the inhibitor efficiency, which is in agreement with our
experimental results [2]. On the other hand, while some previous papers showed that the ELUMO value
is over the Fermi level [31, 56-62], the computational results for the currently studied molecules show
that the ELUMO value has a negative value, thus indicating the capability to accept electrons too.
The theoretical parameters were calculated in vacuo while the electrochemical corrosion
phenomena occur in an aqueous phase. For this reason, the Polarizable Continuum Method (PCM) was
employed in order to consider the solvent effect (water) in the calculations. Table 2 also shows the
results when water is taken into account. The results indicate that the larger the total stabilization
energy the greater the increase in the dipole moment obtained. Solvent effect provides stabilization to
the molecules, vide ΔEsolv in Table 1. Also, the EHOMO values with solvent effect are greater than in
vacuo conditions for all the inhibitors and there is not a proportional effect for the ELUMO values for the
solvent effect systems. Considering the solvent, the ELUMO value drops for the case of 2M5NBI from -
3.28 to -3.55, while the ELUMO value jumped for the 2MI from -1.16 to -0.69 while for the 2MBI and
2M5MBI it remained almost constant. The hardness for 2MI is wider when the solvent effect is taken
into account than in vacuo conditions, for the rest of the inhibitors there are no significant differences,
but the hardness always decrease by solvent effect. In spite of the differences, the same trend was
observed for both approximations.
The results from calculations of hardness and chemical potential in vacuo as well as with the
solvent (water) effect are summarized in Table 2, which reveal that the 2MI exhibits the largest overall
hardness values in both phases, with and without solvent effect. Note that the said hardness increases
for the 2MI due to the solvent effect, while for the rest of the inhibitors it decreases in the presence of
water. However, the chemical potential and the electronegativity decreased in solution in all cases,
with the 2MI being the less electronegative. For the overall electrophilicity and according to the
computed values (see Table 2), the 2MI exhibited a better nucleophilic character, while the 2M5NBI
was lower than the rest. It is well known that cations have electrophilic character, therefore a better
interaction of 2MI, 2MBI and 2M5MBI with the metallic surface would be expected than for 2M5NBI,
see Table 2.
Table 2. Global reactivity descriptors for inhibitors 2MI, 2MBI, 2M5MBI and 2M5NBI in eV, the
values reported are for in vacuo conditions and with solvent effect.
Inhibitor Chemical
Potential (μ)
/eV
Global Hardness
(η)
/eV
Electrophilicity
(W)
/eV
in
vacuo
solv.
Effect
in
vacuo
solv.
Effect
in
vacuo
solv.
effect
2MI -3.13 -2.86 1.96 2.16 2.49 1.89
2MBI -3.32 -3.29 1.93 1.91 2.87 2.84
2M5MBI -3.22 -3.22 1.90 1.89 2.73 2.74
2M5NBI -4.60 -4.58 1.32 1.03 8.01 10.14
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Based on the Pearson theory [14, 24, 25, 59] the fraction of electrons transferred (ΔN) from the
inhibitor molecule to the metallic atom can be calculated. Since a metallic surface and an inhibitor
molecule have different electronegativity, the theoretical reported value [14, 24, 59] for bulk iron χFe is
about 7eV, and the overall hardness ηFe = 0, assuming that the metallic bulk I = A [50, 59], due to the
characteristics of the neutral metallic atoms; where I is the ionization potential and A the electron
affinity. The electronegativity of the inhibitor molecules is lower than the bulk iron: hence, electrons
move from the molecules with lower electronegativity (inhibitor compound) toward that of a higher
value (metal surface) until the equilibrium in chemical potentials is reached. The calculation of ΔN is
obtained from the following formula:
inhFe
inhFeN
2 (12)
The experimental inhibition efficiency and the theoretical value of the fractions transferred
under both conditions, in vacuo and in the presence of water are presented in Table 3. The values of
ΔN indicate the trend within a set of molecules and the highest value of ΔN is related to high inhibitor
efficiency. However, a clearer understanding of this parameter is not yet available. Even more, our
results indicate an opposite behavior when the solvent is accounted for, since the higher ΔN
corresponds to 2M5NBI which is the less effective inhibitor and the lower ΔN value corresponds to
2MI which is the best one. The trend in ΔN in the presence of water follows 2M5NBI > 2M5MBI >
2MBI > 2MI while in vacuo conditions this is not completely clear.
Table 3. Theoretical transferred electron fraction (ΔN) and Back-donation (eV) calculated for both in
vacuo and with solvent effect.
Inhibitor
Molecules
(20 ppm)
Transferred
electrons fraction
Back-donation
/eV
in
vacuo
solv.
Effect
in
vacuo
solv.
Effect
2MI 0.99 0.96 -0.591 -0.639
2MBI 0.95 0.97 -0.480 -0.477
2M5MBI 0.99 1.00 -0.482 -0.478
2M5NBI 0.91 1.17 -0.476 -0.472
According to Gómez et al. [8], an electronic back-donation process might be occurring
governing the interaction between the inhibitor molecule and the metal surface. The concept
establishes that if both processes occur, namely charge transfer to the molecule and back-donation
from the molecule, the energy change is directly proportional to the hardness of the molecule, as
indicated in the following expression [8]:
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4
donationBackE
(13)
The ΔEBack-donation implies that when η > 0 and ΔEBack-donation < 0 the charge transfer to a
molecule, followed by a back-donation from the molecule, is energetically favored. In this context,
hence, it is possible to compare the stabilization among inhibiting molecules, since there will be an
interaction with the same metal, then, it is expected that it will increase as the hardness increases. In
Table 3, the calculated ΔEBack-donation values for both, in vacuo condition and with solvent effect are
included. The order followed is: 2MI > 2MBI > 2M5MBI > 2M5NBI, which indicates that back-
donation is favored for the 2MI, which is the best inhibitor. Finally, an analysis of the charges under
the Mulliken scheme was done to assess changes on the 2MI by addition of substituent groups,
especially over the sulphur atom and the carbon atom (6C) bonded to the substituent group for the
2M5MBI and 2M5NBI compounds.
The atomic NBO charges for the most important centers in vacuo and with solvent effect, are
summarized in Table 4. Redistribution of the NBO charge is obtained by the presence of a substituent;
the most favorable sites for the interaction with the metal surface were the following atoms: 11S, 3C
and 7N for 2MI; 11S, 1C, 4C, 5C and 7N for 2MBI; 11S, 1C, 4C, 5C and 7N for 2M5MBI; and 11S,
4C and 7N for 2M5NBI affected atoms are the 2C, 4C, 5C, 6C, 8C and 11S, because these atoms have
a larger negative charge, which suggests that those active centers with excess charges could act as a
nucleophilic group. As it has been expected, the methyl and nitro group substitution in the 2MBI
molecule, modified the charge distribution of the 2MBI framework, for which when a methyl group is
in the structure the carbons 6C and 2C, are the most affected, while for the nitro group substitution is
the carbon 6C. Generally, its behavior is more clear as a electroattractor group than for the methyl
group as electrodonator, as can be seen in the Table 4.
Table 4. Representative atomic NBO charges for the different inhibitors compounds. See the Figure 1
for the atomic levels.
Inhib
2MI 2MBI 2M5MBI 2M5NBI
q qsolv fi - q qsolv fi
- q qsolv fi
- q qsolv fi
-
7N -0.515 -0.583 -0.031 -0.513 -0.578 -0.024 -0.514 -0.579 -0.022 -0.500 -0.555 -0.027
8C 0.216 0.210 -0.017 0.246 0.247 -0.004 0.245 0.246 -0.004 0.250 0.255 -0.002
9N -0.552 -0.551 -0.053 -0.562 -0.558 -0.033 -0.562 -0.558 -0.029 -0.556 -0.543 -0.042
1C --- --- --- -0.198 -0.225 -0.015 -0.200 -0.227 -0.011 -0.180 -0.190 -0.015
2C -0.066 -0.097 -0.017 0.102 0.114 -0.001 0.120 0.109 -0.001 0.119 0.115 0.000
3C -0.092 -0.092 -0.023 0.122 0.119 -0.003 0.117 0.113 -0.001 0.145 0.164 0.005
4C -0.066 -0.097 -0.017 -0.237 -0.242 -0.006 -0.229 -0.234 -0.003 -0.225 -0.220 -0.003
5C --- --- --- -0.206 -0.226 -0.022 -0.202 -0.220 -0.016 -0.194 -0.197 -0.017
11S -0.006 0.025 -0.076 0.044 -0.558 -0.076 0.042 0.011 -0.071 0.065 0.044 -0.077
15H 0.462 0.407 -0.343 0.409 0.465 -0.316 0.409 0.465 -0.319 0.217 0.247 -0.117
19H 0.162 0.204 -0.132 0.164 0.217 -0.165 0.163 0.216 -0.145 0.223 0.168 -0.198
6C --- --- --- -0.217 -0.226 -0.021 -0.045 -0.050 -0.015 0.068 0.064 -0.020
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3738
(a)
(b)
(c)
(d)
Figure 2. Isosurfaces of the Fukui function distribution for the different inhibitors conditions. The
isosurface value is 0.02.
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In Figure 2, a schematic representation of the Fukui functions obtained from the eq. 10 is
showed. The isosurface representation is for )(rf Ns , but for the case of )(rf Ns
, the representation is
very similar, as the can be noticed in eq. 10. The difference between these functions is only the sign,
which turn out to be positive or negative according to the function. Under this scheme, the result
suggests that this group of molecules has the ability to act as charge donor or acceptor, since the Fukui
functions, )(rf Ns and )(rf Ns
, have a high contribution over the complete molecule, then a dual
behavior could be expected that agrees too with the back-donation found; both results are in line with
the experimental observation [2], since the 2MI proved to be a mixed-type inhibitor, as it affected both
anodic and cathodic process. In fact, if the isosurfaces of the group of inhibitors are compared, Figure
2, then 2MI has an important contribution in the region over the sulphur atom than the rest of the
inhibitors. This result suggests that maybe the interaction with the metallic surface is through this atom
mainly. Although the rest could contribute to the interaction too; even though the hardness is the
greatest for the 2MI than for the rest, but the ELUMO value has a negative value, which means certain
capability to accept electrons.
It is important to stress out that Ivanova and Pindeva [63] using solid-state linear dichroic
infrared (IR-LD) spectral analysis and ab initio calculations have shown that the protonation of
benzimidazoles and 1,2,3-benzotriazoles can be occurring under acidic conditions. Thus it will very
important to determine experimentally the acidity constant of each of the molecules considered here in
order to carry out the theoretical analysis, in its case, the appropriate protonated or unprotonated
molecules: this study is currently being carried out in our laboratory.
4. CONCLUSIONS
As previously shown experimentally (R. Álvarez-Bustamante, G. Negrón-Silva, M. Abreu-
Quijano, H. Herrera-Hernández, M. Romero-Romo, A. Cuán, M. Palomar-Pardavé. Electrochim. Acta,
54, (2009) 539) the IE order for the molecules tested was 2MI > 2MBI > 2M5MBI > 2M5NBI. 2MI
showed the best corrosion efficiency as inhibitor in spite of the lack of multiple bonds, -electrons
conjugated and polar part. The 2MI compound reached a relative maximum inhibiting efficiency of
98.5 % at 25 ppm concentration, following the Langmuir isotherm with an adsorption standard Gibbs
Free Energy difference (G0
ads) of -26.8 kJ mol-1
. The 2MI IE was measured as a function of time,
where the IE decreases linearly with time displaying a slope of -0.03h-1
, after 800 evaluation hrs. After
this time, the 2MI IE falls to 70 % and it remains constant up to 1200 hrs. It is shown that this
compound can affect both the anodic and cathodic processes, thus it can be classified as a mixed-type
inhibitor. The values obtained for the standard Gibbs free energies of adsorption and the negative
values of EHOMO corroborates that physical adsorption of the inhibitors tested is effective onto the metal
surface.
In this work it was found a quantitative relationship between EHOMO, hardness (η), electron
charge transfer (ΔN), Electrophilicity (W), Back-donation (ΔEBack-donation) and the inhibition efficiency
for the tested compounds. All these parameters represent better the actual experimental situation. As
comparing the 2MI with the other inhibitors, it exhibits a better nucleophilic character; the highest
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3740
EHOMO and the highest electron charge transfer (ΔN), hence underlining its good ability as an electro-
donator. It too has the largest hardness and therefore a favorable back-donation charge. These results
were found for both phases, namely with and without solvent effect.
According to the Fukui functions analysis, a dual behavior could be expected, since the
contributions of both )(rf Ns and )(rf Ns
, have a large contribution over the complete molecule
being largest for the 2MI with an important contribution in the region over the sulphur atom than the
rest of the inhibitors, thus suggesting that the interaction with the metallic surface could be through this
atom mainly.
The substituting groups as electrondonors or electroattractors do not have an important
influence on the behavior over the 2MBI inhibitor, because from the experimental and theoretical point
of view, the 2MBI is better inhibitor than the 2M5MBI or 2M5NBI, no matter the substituent’s
character.
Although a number of satisfactory quantum chemical parameters showed a good correlation
with the inhibition efficiency of various inhibitors, there is still a lack of a simple correlation between
some others like dipole and ELUMO with the inhibition efficiencies.
ACKNOWLEDGEMENTS
MAQ expresses his gratitude to CONACyT for his Ph.D. studentship (162447). MRR and MPP would
like to thank CONACYT for project 22610714, and HHH to ICyTDF for his postdoctoral fellowship.
MRR, AC, GNS, HHH and MPP, gratefully acknowledge the SNI for the distinction of their
membership and the stipend received. Authors also wish to express their gratitude to Departamento de
Materiales at UAMA for the support given through projects 2261203, 2261204, 2261205 and to
Departamento de Ciencias Básicas for project 2232506. This work was done in partial fulfillment of
MAQ.’s Ph.D. requirements.
References
1. P. Morales-Gil, G. Negrón-Silva, M. Romero-Romo, C. Ángeles-Chávez, M. Palomar-Pardavé,
Electrochim. Acta 49, (2004) 4733.
2. R. Álvarez-Bustamante, G. Negrón-Silva, M. Abreu-Quijano, H. Herrera-Hernández, M. Romero-
Romo, A. Cuán, M. Palomar-Pardavé. Electrochim. Acta, 54, (2009) 539.
3. S. Corona-Avendaño, G. Alarcón-Ángeles, G.A. Rosquete-Pina, A. Rojas-Hernández, A.
Gutiérrez, M.T. Ramírez-Silva, M. Romero-Romo, M. Palomar-Pardavé, J. Phys. Chem. B, 111
(2007) 1640.
4. L.H. Mendoza-Huizar, M. Palomar-Pardavé, J. Robles, Electrochim. Acta (2001) 2749.
5. J.A. Cobos-Murcia, L. Galicia, A. Rojas-Hernández, M.T. Ramírez-Silva, R. Álvarez-Bustamante,
M. Romero-Romo, G. Rosquete-Pina, M. Palomar-Pardavé, Polymer 46 (2005) 9053.
6. Y. Yan,W. Li, L. Cai, B. Hou, Electrochim. Acta 53 (2008) 5953.
7. K.F. Khaled, Electrochim. Acta 53 (2008) 3484.
8. B. Gómez, N. V. Likhanova, M. A. Domínguez-Aguilar, R. Martínez-Palou, A. Vela, and J. L.
Gázquez, J. Phys. Chem. B 110 (2006) 8928.
9. O. Olivares, N.V. Likhanova, B. Gómez, J. Navarrete, M. E. Llanos-Serrano, E. Arce, J. M. Hallen,
Appl. Surf. Sci. 252 (2006) 2894.
10. P. Bothi, M. Gopalakrishnan, Mater. Lett. 62 (2008) 113.
11. Y.S. Li, A. Ba, M.S. Mahmood, Electrochim. Acta 53 (2008) 7859.
Int. J. Electrochem. Sci., Vol. 6, 2011
3741
12. P.C. Okafor, M.E. Ikpi, I.E. Uwah, E.E. Ebenso, U.J. Ekpe, S.A. Umoren, Corros. Sci. 50 (2008)
2310.
13. P. de Lima-Neto, A.P. de Aráujo, W.S. Aráujo, A.N. Correia, Prog. Org. Coat. 62 (2008) 344.
14. L.R. Chauhan, G. Gunasekaran, Corros. Sci. 49 (2007) 114.
15. T. Fallavena, M. Antonow, R.S. Goncalves, Appl. Surf. Sci. 253 (2006) 566.
16. M.I. Awad, J. Appl. Electrochem. 36 (2006) 1163.
17. M. Zubielewicz, W. Gnot, Prog. Org. Coat. 49 (2004) 358.
18. G. Gunasekaran, L.R. Chauhan, Electrochim. Acta 49 (2004) 4387.
19. E. Stupnisek-Lisac, A. Gazivoda, M. Madzarac, Electrochim. Acta 47 (2002) 4189.
20. A.V. Shanbhag, T.V. Venkatesha, R.A. Prabhu, R.G. Kalkhambkar, G.M. Kulkarni, J. Appl.
Electrochem. 38 (2008) 279.
21. E. Machnikova, K.H. Whitmire, N. Hackerman, Electrochim. Acta 53 (2008) 6024.
22. S.A. Ali, H.A. Al-Muallem, M.T. Saeed, S.U. Rahman, Corros. Sci. 50 (2008) 664.
23. S.A. Ali, M.T. Saeed, S.U. Rahman, Corros. Sci. 45 (2003) 253.
24. D. Wahyuningrum, S. Achmad, Y.M. Syah, Buchari, B. Bundjali, B. Ariwahjoedi, Int. J.
Electrochem. Sci. 3 (2008) 154.
25. M. Benmessaoud, K. Es-Salah, N. Hajjaji, H. Takenouti, A. Srhiri, M. Ebentouhami, Corros. Sci.
49 (2007) 3880.
26. S.A.M. Refaey, F. Taha, A. M. Abd El-Malak, Int. J. Electrochem. Sci. 1 (2006) 80.
27. L. Larabi, O. Benali, S.M. Mekelleche, Y. Harek, Prog. Org. Coat. 253 (2006) 1371.
28. N. Kaniskan, C. Ogretir, J. Mol. Struct. (Theochem) 584 (2002) 45.
29. M. Özcan, İ. Dehri, Progress in Organic Coatings 51 (2004) 181.
30. D.Q. Zhang, L.W. Gao, G.D. Zhou Corros. Sci. 46 (2004) 3031
31. G. Gao, C. Liang, Eletrochim. Acta 52 (2007) 4554.
32. M. J. Frisch, G. W.Trucks, H. B.Schlegel, G. E.Scuseria, M. A.Robb, J. R. Cheeseman, J. A., Jr.
Montgomery, T.Vreven, K. N.Kudin, J. C.Burant, J. M. Millam, S. S.Iyengar, J. Tomasi, V.
Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M.
Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.
E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K.
Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D.
Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J.
V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P.
Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A.Al-Laham, C. Y. Peng, A.
Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J.
A. Pople, Gaussian 03, Revision B.3; Gaussian, Inc.: Pittsburgh, PA, 2003.
33. A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
34. C. Lee, W. Yang, R. G.Parr, Phys. Rev. B 37 (1988) 785.
35. S. Miertus, E. Scrocco, J. Tomasi, J. Chem. Phys. 55 (1981) 117.
36. V. Barone, M. Cossi, J. Tomasi, J. Chem. Phys. 107 (1997) 3210.
37. M.A. Thompson, M.C. Zerner, J. Am. Chem. Soc. 113 (1991) 8210.
38. M.A. Thompson, E.D. Glendening, D. Feller, J. Phys. Chem. 98 (1994) 10465.
39. M.A. Thompson, G.K. Schenter, J. Phys. Chem. 99 /1995) 6374.
40. M.A. Thompson, J. Phys. Chem. 100 (1996) 14492.
41. R.G. Parr, W. Yang, Density functional theory of atoms and molecules. New York: Oxford
University Press; 1989]:
42. M. Galván, A. Vela, J. L. Gázquez, J. Phys. Chem. 92 (1988) 6470.
43. R.G. Parr, L. Szentplay, S. Liu, J. Am. Chem. Soc. 121 (1999), 1922.
44. R.G. Parr, W. Yang, P.W. Ayers, M. Levy, Theor. Chem. Acc. 103 (2000), 353
Int. J. Electrochem. Sci., Vol. 6, 2011
3742
45. K. Fukui, The world of quantum chemistry Daudel and Pullman ed USA. Proceedings of the first
international congress of quantum chemistry held at menton 1974. p. 113–141.
46. M. Galván, R. Vargas, J. Phys. Chem. 96 (1992) 1625.
47. J. Cioslowsky, M. Martinov, S.T. Mixon, J. Am. Phys. Chem. 97 (1993) 10948.
48. F. Profit, R. Vivas-Reyes, A. Peeters, C. Alsenoy, P. J. Geerling, Comp. Chem. 24 (2003) 463.
49. W. Yang, W.J. Mortier, J. Am. Chem. Soc. 108 (1986) 5708.
50. F. Bentiss, M. Lebiri, H. Vezin, M. Lagrenee, J. Matt. Chem. Phys. 87 (2001) 18.
51. H. Wang, X. Wang, L. Wang, A. Liu, J. Mol. Model 13 (2007) 147.
52. N.K. Allan, J. App. Surf. Sci. 253 (2007) 4570.
53. M. Sahin, G. Gece, F. Kaerci, S. Bligic, J. Appl. Electrochem. 38 (2008) 809.
54. V.S. Sastri, J. R. Perumareddi, Corros. Sci. 50 (1994) 432.
55. J. Cruz, E. García-Ochoa, M. Castro. J. Electrochem. Soc. 150 (2003) B26.
56. F. Bentis, M. Traisnel, H. Vezin, H.F. Hildebrand, M. Lagrenée. Corros. Sci. 46 (2004) 2781.
57. H. Wang, X. Wang, H. Wang, L. Wang, A. Liu, J. Mol. Model 13 (2007) 147.
58. N.K. Allam, Appl. Surf. Sci. 253 (2007) 4570.
59. L.M. Rodríguez-Valdez, W. Villamisar, M. Casales, J.G. González-Rodríguez, A. Martínez-
Villafañe, L. Martínez, D. Glossman-Mitnik. Corros. Sci. 48 (2006) 4053.
60. V. F. Ekpo, P. C. Okafor, U. J. Ekpe, E. E. Ebenso, Int. J. Electrochem. Sci., 6 (2011) 1045.
61. N.O. Eddy, B. I. Ita , N.E. Ibisi, E.E. Ebenso, Int. J. Electrochem. Sci., 6 (2011) 1027.
62. E.E. Ebenso, I.B. Obot, Int. J. Electrochem. Sci., 5 (2010) 2012.
63. B.B. Ivanova, L.I. Pindeva, J. Mol. Struct., 797. (2006) 144.
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